Parameters influencing thermomechanical fatigue of a directionally-solidified Ni-base superalloy

Abstract

Thermomechanical fatigue (TMF) is a low cycle fatigue process in which material life is correlated to the mechanical strain amplitude. However, it is well known that several other factors influence this life. This paper examines several of these parameters and their influence on life using experiments conducted on a second generation directionally-solidified (DS) Ni-base superalloy. The parameters considered include the influence of the temperature extremums (T max of either 750 or 950 • C and T min of either 100 or 500 • C), strain ratio (R ), the strain-temperature phasing (in-phase (IP) and out-of-phase (OP)), the influence of dwells at the high temperature end of the cycle resulting in a creep-fatigue (CF) interaction, and material anisotropy associated with the grain growth direction (longitudinal versus transverse). Results suggest that the phasing has a primary role in controlling the mechanism of degradation. IP TMF is dominated by crack formation in volumes surrounding debonded carbides for both continuously cycling (CC) and CF at 950 and 750 • C, while OP TMF is dominated by surface oxidation and repetitive cracking of the oxide that reforms at the crack tip at 950 • C. Decreasing the T max to 750 • C the environmental and creep effects are reduced resulting in virtually pure fatigue exposure under OP conditions. With decreasing T min from 500 • C to 100 • C was observed an increase in inelastic strain amplitude and 1 corresponding decrease in life. Variations in R were found to have no significant influence on life or stabilized stress behavior. TMF loading in the transverse orientation resulted in life reductions over the longitudinal orientation due to cracks propagating in a transgranular manner. Lastly, only in material exposed to CF with a T max of 950 • C rafting of the γ precipitates was observed.

Influence of coarsened and rafted microstructures on the thermomechanical fatigue of a Ni-base superalloy

“…14. As discussed in a companion paper [27], a reduction in life is observed when the T min is reduced which the mechanical strain amplitude remains constant. Under CC conditions, the life of the P-raft microstructure is further reduced by a factor of two or greater when compared to the virgin material for both T min .…”

“…4. For both microstructures, life is reduced with a reduction in the minimum cycle temperature (T min ) and further reduced with inclusion of a dwell [27]. When comparing the same test conditions, the stressfree coarsened microstructure under 500-750°C CC TMF resulted in a 85% life reduction, while the TMF cycle with the lower T min , 100-750°C, exhibited a 65% life reduction when compared to the virgin material responses.…”

“…The criterion for crack initiation (material life) was defined as a 5% load drop from the peak of the stabilized hysteresis response. This failure (life) criterion corresponds to a crack in the range of 0.5 and 1.0 mm [26,27]. The mechanical strain amplitude was chosen to target conditions where the macroscopic hysteresis response exhibited minimal cyclic inelastic strain.…”

Crystal Viscoplasticity of a Ni-Base Superalloy in the Aged State

“…Further, the model captures the peak stresses within 10% (50-80 MPa) of the experimental measured response of the TMF behavior for which the validation was carried out. However, when accounting for statistical material behavior in the cyclic response, this can be considered within the realm of reasonable variation for Ni-base superalloys [37].…”

Crack Initiation and Propagation during Thermal-Mechanical Fatigue of IN792: Effects of Dwell Time

“…Although the crack initiation sites can be related to oxides forming from the surface, the fact that the majority of MC carbides are cracked at the IP-tests is the prevailing observationthe formation of voids at these particular interfaces in the IP-tests dramatically decreases the crack initiation time of the alloy. MC carbides have been reported to serve as crack initiation sites in previous studies on polycrystalline superalloys particularly during isothermal fatigue studies [5,6,7,8,9].…”